| Oak Grove Folk Music Festival | Staunton | VA | United States | August | Bluegrass |
| Oceans Calling Festival | Ocean City | MD | United States | September | Indie, Pop, Rock |
| Off Beat Music Festival | Reno | NV | United States | October | Indie |
| Ogden Music Festival | Ogden | UT | United States | June | Bluegrass |
| Ohana Festival | Dana Point | CA | United States | September | Alternative, Country, Folk, Indie, Rock |
| OKC Roots Music Festival | Oklahoma City | OK | United States | October | Americana, roots, rock, blues, and folk, featuring singer-songwriter artists. |
| Oklahoma’s International Bluegrass Festival | Guthrie | OK | United States | October | Bluegrass |
| Ol’ Front Porch Music Festival | Oriental | NC | United States | October | Bluegrass |
| Old Fiddler’s Convention | Galax | VA | United States | August | Bluegrass |
| Old Settler’s Music Festival | Tilmon | TX | United States | April | Bluegrass |
| Old Time Music Ozark Heritage Festival | West Plains | MO | United States | September | Bluegrass |
| Old Town Music and Arts Festival | Williamsburg | OH | United States | August | Bluegrass |
| Olde Dominion Bluegrass Festival | Chatham | VA | United States | November | Bluegrass |
| Oldtone Roots Music Festival | Hillsdale | NY | United States | September | Bluegrass |
| Opelika Songwriters Festival | Opelika | AL | United States | May | Singer-Songwriter, Americana, Folk and Country |
| Orange Army Bluegrass Festival | Flemingsburg | KY | United States | August | Bluegrass |
| Oregon Jamboree | Sweet Home | OR | United States | August | Country |
| Osceola Music Festival | Osceola | IN | United States | August | Bluegrass |
| Ossippee Valley Music Festival | South Hiram | ME | United States | July | Bluegrass, but also features roots, Americana, folk, blues, and world music. |
| Outlandia Festival | Bellevue | NE | United States | August | Indie, Rock |
| Outlaw Country Cruise | Miami | FL | United States | February | Country, Rock |
| Outside Lands | San Francisco | CA | United States | August | Alternatic, Electronic, Hip Hop, Inide, Rock |
| Oyster Ridge Music Festival | Kemmerer | WY | United States | July | Bluegrass |
| Ozark Folk Festival | Mountain View | AR | United States | September | Folk music, with elements of Americana, singer-songwriter, and roots music |
